Information is handled in digital computers in the form of words or bytes, where a byte consists of a fixed number of binary digits or bits. When information must be transferred between two locations within the computer, the byte is usually transferred in parallel over a multi-wire bus connecting the two locations.
When the same byte must be sent to a location outside the computer, it is often sent one bit at a time or serially. The advantages of serial transmission over parallel transmission include the lower cost of a single wire compared to a multi-wire cable, the fewer number of connectors required and the smaller size and weight of the single wire cable.
Each byte must be serialized (converted to a string of bits) at the transmitting apparatus and then deserialized (reassembled into byte form) at the receiving apparatus. The receiving apparatus must be able to identify the beginning and end of each byte in the serial stream of data. This can be done with special bit patterns. Since the special patterns have no information content, the effective transmission rate is reduced. Also, when information is being transferred between computers that operate at comparable speeds, serial data must be transmitted at a rate equal to the internal computer transfer rate multiplied by the sum of the number of bits per byte plus the number of bits required to define any special patterns. Obviously, the amount of data that can be transmitted between two computers in a given unit of time is considerably less where data is transmitted serially rather than in parallel.
There has been a good deal of interest in frequency or wavelength division multiplexing as a technique which might be used to accomplish parallel data transmission on a serial transmission link such as single wire or a single optical fiber. In an optical wavelength division multiplexing (or WDM) system, the individual data bits in a byte are applied to different encoders in a set of optical encoders. The encoders modulate each bit or channel at a different optical frequency or wavelength. The modulated optical signals are merged and transmitted through a single fiber to a receiving location. At the receiving location, the light beam is separated, using optical filters or gratings, into its bit signal components to re-establish the original byte form of the data.
One known problem in using wavelength division multiplexing in an optical fiber system is that an optical fiber is a dispersive medium which tends to delay optical signals at some frequencies more than optical signals at other frequencies. Since a byte of WDM data consists of signals at different frequencies or wavelengths, the effect of dispersion is to delay some bits in the byte more than other bits in the same byte. The longer the optical fiber, the greater the relative delay which may occur between the individual bits in a WDM byte of data. The net effect of the frequency-related dispersion is that not all bits in a single WDM byte may arrive at a receiving location at the same time even though all may have been transmitted at the transmitting location at the same time. The arrival of bits in a given byte at different times is referred to as bit skew.
If the transmission rate is not too high or the length of the transmission link is not too long, the amount of bit skew may remain within acceptable limits. However, for high transmission rates and/or longer transmission links, the skew may become large enough that bits in one byte at least partially overlap bits in another earlier or later byte.
The dispersion characteristics of common transmission mediums, such as optical fibers, are well known for wavelengths that are of interest for data transmission. If the exact length of a transmission path is known, the retardation or delay of a bit transmitted at a particular wavelength can be readily calculated.
Unfortunately, the exact length of transmission links in practical, commercial communications systems is not normally known. The exact length of a transmission link depends on the manner in which it is installed and will vary from one installation to the next, depending on the installer's techniques. Even if the exact length is known initially, it can change if connectors and/or sections of fiber must be replaced after installation.
Because the exact length of an optical fiber is not normally known, other solutions for dealing with the problem of bit skew have been proposed.
One way to deal with bit skew is to limit the transmission rate to avoid overlap between successive bytes. This, of course, limits the maximum permissible data transfer rate, which is undesirable as being contrary to the basic reasons for using wavelength division multiplexing as a data transmission technique.
Another potential technique for dealing with the problem of bit skew is to require that the transmitting system head each message with a series of special bit patterns which, when detected, can be used to determine the time of arrival of the bits in each of the parallel channels. The special bit patterns have the disadvantage that they represent "overhead" which reduce the effective throughput of the system.